1. Field of the Invention
The present invention relates to amplifiers, and more particularly to a high voltage differential amplifier implemented using low voltage devices.
2. Description of the Related Art
Server computer systems and the like are often used to support multiple users and/or execute important functions. For many such applications, it is desired that the server remain active to provide continuous and uninterrupted service even during maintenance operations. Maintenance includes repairs, exchanges, upgrades, etc., which should be performed without shutting down the server. Electronic “hot plug” or “hot swap” technology has been developed to meet this need. Hot plug devices and controllers allow boards, disk drives, and other peripherals to be removed and replaced without shutting down system power. Hot plug devices and controllers offer one or more of a variety of features, including electronic circuit breakers, fault isolation, over-current protection, current regulation, UV and over-voltage protection, soft-start functions, fault reporting, etc. Hot plug devices and controllers must be capable of safely switching the voltage levels employed by the underlying computers and peripheral devices while system power is active.
Semiconductor processes continue to develop and have led to smaller feature sizes, higher yields and thus significantly reduced costs. Technological advances have decreased the semiconductor process platforms to sub-micron levels, including the 0.13 micron devices and more recent 90 nanometer (nm) devices. One particular technology is Bipolar Complementary Metal-Oxide Semiconductor (BiCMOS) in which bipolar type devices are implemented using CMOS processes. BiCMOS is a merger of bipolar and CMOS technologies in the same substrate. This merger combines the small feature size of CMOS (which is very advantageous for digital circuits) and substantially all the advantages of bipolar in analog circuits. The advantages include higher precision, good transconductance, good matching and thus low off-set voltages at differential amplifiers, etc. These features make BiCMOS the ideal technology for mixed-signal VLSI (very large scale integration) applications.
It is desired to use the improved semiconductor processes to implement hot plug devices and controllers. Yet hot plug devices and controllers must be capable of switching all voltage levels of the peripheral devices. Although the voltage levels required for signal processing devices and the like have enabled decreased voltage levels from 5 volts (V) to 3.3 V and 2.5 V and even lower, peripheral devices often include older technologies and/or high power devices, which employ higher voltage levels such as 12 V or more. One disadvantage of the newer technologies operable at lower voltage levels is the concomitant reduced level of breakdown voltage. BiCMOS devices, for example, have a breakdown voltage at or near the 6 V level, which are incapable of switching higher voltage levels.
Another technology, called double diffused metal oxide semiconductor (DMOS), is available for high voltage and/or high power applications. A DMOS transistor essentially is a special type of MOS transistor manufactured by making an additional lightly doped diffusion on the drain side of the transistor. The additional diffusion causes the effective channel length to be shorter than drawn length. The additional diffusion increases conductivity so that the DMOS transistor has a lower “on” resistance. The additional, lightly doped region in the channel causes the drain-to-source breakdown voltage to increase, so that it can withstand higher voltages (20–30 V) before it breaks down. Thus, a DMOS transistor can handle larger currents for a given gate voltage, which makes it very suitable for switching applications. DMOS transistors are high voltage devices that are suitable to be used as switches or cascading devices, but are not suitable for signal processing amplification.
It is desired to employ the newer technologies for hot plug devices and components, while also supporting the necessary functions of switching higher voltages levels including 12 V or more.